Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In direct thermal thermography a visible image pattern is formed by image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density. Such recording materials become photothermographic upon incorporating a photosensitive agent which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density.
Research Disclosure number 17029, published in June 1978, gives a survey of different methods of preparing organic heavy metal salts in section II. The invention examples of U.S. Pat. No. 5,380,635 and U.S. Pat. No. 5,434,043 describe the production of organic silver salts using fatty acids of the type HUMKO Type 9718 & Type 9022 from WITCO Co., which contain according to the manufacturer's catalogue a mixture of different fatty acids, in connection with their use in photothermographic recording materials. DE-OS 27 21 828 discloses a thermally developable light-sensitive material, consisting of a support, which contains thereon or in one or more layers at least (a) an organic silver salt, (b) a photocatalyst and (c) a reducing agent, wherein the organic silver salt (a) contains at least a silver salt with an uneven number of 21 or more carbon atoms; and examples with mixtures of two and three organic silver salts of monocarboxylic acids precipitated together, but all with 20 are more carbon atoms.
U.S. Pat. No. 5,677,121 discloses a heat-developable silver halide infrared ray-sensitive material comprising a support having on one side of the support an emulsion layer containing a binder, a nonsensitive silver salt, a reducing agent for silver ion and silver halide grains spectrally sensitized at a wavelength within the region of from 750 to 1400 nm, wherein the nonsensitive silver salt comprises a mixture of silver salts of at least three organic carboxylic acids, one of the acids is behenic acid, and the content of the behenic acid in the acids is from not less than 35 to less than 90 mol %.
However, technology from photothermographic materials on the basis of an organic silver salt, silver halide and a reducing agent is not readily extrapolatable to substantially light-insensitive thermographic recording materials on the basis of an organic silver salt and a reducing agent, since thermographic recording materials are subjected to image-wise heating whereas photothermographic materials are subjected to image-wise exposure and overall heating and much stronger reducing agents are used in thermographic recording materials than in photothermographic recording materials. Furthermore, thermographic recording materials are heated for much shorter times, typically 10 to 20 ms, during thermal development in thermographic printing than photothermographic recording materials, for which 10 s is an average heating time. Such shorter heating times make it difficult to obtain neutral image tones.
EP-A 730 196 discloses a heat-sensitive recording material suited for use in direct thermal imaging and having image-stabilization properties which material contains in a binder on a support (i) a substantially light-insensitive organic silver salt capable of thermally activated reduction to silver in thermal working relationship with (ii) at least one reducing agent capable of reducing the substantially light-insensitive organic silver salt when thermally activated, characterized in that the recording material contains in admixture with the reducing agent(s) at least one colourless photo-oxidizing substance that on exposure to ultraviolet radiation yields free radicals capable of inactivating the reducing agent(s) by oxidation, thereby rendering the reducing agent(s) incapable of reducing the organic silver salt to silver. Furthermore, in sub-claims the organic silver salt is silver palmitate, silver stearate or silver behenate or mixtures thereof. However, the efficacy of such physical mixtures is not exemplified. Physical mixtures in which each component forms a separate phase cannot be equated with mixed crystals in which the components together form a single phase.
Prior art substantially light-insensitive black and white thermographic recording materials exhibit an insufficiently neutral image colour. This is particularly important for thermographic recording materials for medical diagnostic applications for which image tone requirements are particularly severe, particularly at low optical densities. Prior art thermographic recording materials coated from solvent exhibit image tone closer to these requirements than those coated from aqueous media, although the latter are producible using much more environmentally friendly coating processes.